1. Field of the Invention
This invention relates to a method for the manufacture of a III-V group compound semiconductor single crystal by the liquid-encapsulated Czochralski method (LEC method).
2. Description of Prior Art
Such III-V group compound semiconductors as gallium arsenide (GaAs), gallium phosphide (GaP), indium antimonide (InSb), and indium phosphide (InP) have characteristics such as high electron mobility, ready emission of light, capability of detecting light, and ability to operate even at high temperatures and, consequently, find extensive utility as materials for microwave transistors, high-speed integrated circuits, solar cells, and photo-electron devices. Since the single crystal of GaAs, among the III-V group compound semiconductors, possesses electron mobility 5 to 6 times as high as the electron mobility of the single crystal of silicon, development of a GaAs integrated circuit is now being promoted energetically. The single crystal of InP exhibits high sensitivity to the low-loss band of the optical fiber used in optical communication and is attracting attention as a material for the future optical communication.
In order for the single crystal of GaAs to be used as a crystal substrate for an integrated circuit, it is required to possess high quality free from physical faults such as dislocation and lattice defect and from chemical faults, enjoy high intracrystalline uniformity, and enable production of a large circular wafer. As a method capable of producing the single crystal of GaAs satisfying these requirements, there may be cited the high-pressure LEC method. This high-pressure LEC method accomplishes the formation of a cylindrical single crystal of GaAs by using a low-melting glass like boron oxide (B.sub.2 O.sub.3) as an encapsulant, melting GaAs under high pressure, bringing a seed crystal into contact with the resultant fused GaAs layer, and pulling the seed crystal from the fused GaAs layer while rotating it.
The temperature of the fused layer of raw material for the crystal in the process of growth into crystal is a delicate factor. In the first place, the fact that at the time the seed crystal brought into contact with the fused layer of raw material for crystal begins to be pulled from the fused layer, the fused layer is at the temperature optimum for the speed at which the growing crystal is pulled constitutes one important determinant for successful manufacture of the single crystal of high quality.
In accordance with the Czochralski method, for example, which is effected for the growth of the single crystal of silicon without use of any liquid encapsulant, it has been customary to produce the single crystal of high quality with high repeatability by following the change in weight of the seed crystal and, at the same time, consulting the optimum seed crystal contact temperature curve prepared in advance on the basis of an optimum temperature in the state of mere contact between the seed crystal and the fused layer of raw material for the crystal. In accordance with the method for the production of a III-V group compound semiconductor single crystal by the use of a liquid encapsulant, however, since the seed crystal is susceptible to the buoyancy originating in the liquid encapsulant, the signal indicating detection of a change in weight of the seed crystal has low reliability because of its high noise content. Besides, the method which uses the optimum seed crystal contact temperature curve based on the optimum temperature in the state of mere contact between the seed crystal and the fused layer of raw material necessitates correction of the actual seed crystal pulling rate with reference to the temperature curve. It is extremely difficult to have the temperature of the interface of the fused layer of raw material for the crystal adjusted at all times accurately within only .+-.0.5.degree. C. of the optimum level relative to the crystal pulling rate. As a result, in the adjustment of the temperature of the fused layer of raw material for the crystal to be made while the seed crystal is being brought into contact with the fused layer of raw material and it is being pulled out of the fused layer there is no alternative but to rely upon the skill of an operator for temperature adjustment. The adjustment performed in this manner is not repeatable and has so far formed a bottleneck in the complete automation of the manufacture of single crystal.